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Property Match Expressions


A Property Match Expression is a special type of Expression that is used to select a set of counters to be operated on (Module, Map, Deck or Counter Global Key Commands) or to test if some condition is true before taking action (Trigger Action, Restrict Commands). Such expressions are so-named because their conditions usually check the values of Properties.

Just as there are two type of Expressions, there are two types of Property Match Expressions. BeanShell Expressions are identified by being surrounded by curly braces {}. They are very powerful and can be arbitrarily complex. Old-Style Expressions are identified by not being surrounded by the curly braces {}. Old-Style Expressions are less powerful than BeanShell expressions and are supported to provide compatibility with earlier versions of Vassal. If you are learning VASSAL for the first time, you will mainly want to learn about BeanShell expressions.

Comparing Properties For Global Key Commands

When Property Match Expressions are used in a Global Key Command trait attached to a piece, any references to Properties normally refer to the properties of the piece being checked to see if it matches, i.e. the potential "target" piece, rather than the piece that is issuing the Global Key Command. However, there is a special format which can be used to match properties in the potential target pieces against properties in the issuing piece that is generating the command. Any Property name enclosed in $ signs (e.g. $nation$) will be evaluated against the piece issuing the Global Key Command and replaced before the property Match is performed against other pieces. Because the $..$ notation performs an immediate substitution, this means that string values will often need to be put inside of quotation marks. Thus, the matching expression { nation == "$nation$" } will compare the value of the nation property of the target piece (left side of equation in this case) to the value of the nation property of the issuing piece (right side of the equation in this case).

BeanShell Property Match Expressions

BeanShell Property Match Expressions use the same syntax as BeanShell Expressions . They use the full syntax and capabilities of the BeanShell language and can be arbitrarily complex:

    {MaxHeight < (CurrentHeight + Growth) && Age > $myAge$}

    {Nation == (unitType=="Infantry" ? unitNation : formationNation)}

Java String and Math Methods

Java string and arithmetic methods can also be used to evaluate properties:

{ GetProperty("AP_Objectives_" + Variant).contains(LocationName) ? 1 : 0 }

Checks if a list of objectives contains the location name string.

{ LocationName.length() }

The length of the LocationName string

{ Integer.parseInt((LocationName).replaceAll("[^0-9]","")) }

Value of any number in LocationName, after removing all non-numeric characters

{ Math.abs(MaybeNegative) }

The absolute value of the numeric property MaybeNegative

{ Math.max(OneValue, GetProperty("Value"+WhichValue)) }

The larger of two numeric properties

Old-style Property Match Expressions PropertyMatchExpression2
Old-style Property Match Expressions are not surrounded by the curly braces and have a much-restricted syntax compared to their BeanShell equivalent. They are supported to maintain compatibility with older VASSAL modules — designers of new modules should prefer the more powerful BeanShell expressions.#

  • The left-hand side of each comparison MUST be a property name.

  • The right-hand side of each comparison MUST be a string NOT surrounded by quotes.

  • The Equals operator is = not ==.

  • Parentheses () may NOT be used to create more complex expressions.

  • Operators and functions may NOT be used to create more complex expressions.